The present invention relates generally to transport of a wet gas and, more particularly, to a process for converting a hydrocarbon stream containing a gas phase and a liquid phase to a stable gas hydrate slurry and transporting the resulting gas hydrate slurry through a subsea gas pipeline.
Gas streams, such as natural gas, produced from offshore wells are often transported under high pressure from the subsea well head through a subsea gas transport pipeline to a surface locale, such as a host platform or other surface facility, for processing or distribution. In many cases a liquid aqueous phase and a gaseous hydrocarbon phase coexist in the produced gas stream, which is fed from the well head into the subsea gas transport pipeline. Produced gas streams, which contain a liquid aqueous phase and a gaseous hydrocarbon phase, are termed wet gas.
At the temperature and pressure conditions frequently encountered in subsea gas transport pipelines, the gas and liquid phases of a wet gas readily react to form solid gas hydrates. The solid gas hydrates can undesirably occlude the subsea gas transport pipeline by building up on the interior walls of the pipeline, ultimately aggregating into a plug or blockage. The present invention recognizes a need for a cost-effective solution to the problem of transporting a wet gas via subsea gas transport pipeline while avoiding substantial occlusion of the pipeline with aggregated solid gas hydrates. As such, the present invention is a process for efficiently transporting a wet gas via a subsea gas transport pipeline. The present process converts the wet gas to a gas hydrate slurry comprising solid gas hydrate particles suspended in a continuous liquid phase before feeding the wet gas into the subsea gas transport pipeline, which preempts the formation and aggregation of solid gas hydrates in the pipeline during transport of the wet gas.
Several methods are known in the prior art for producing solid gas hydrates, but none are deemed satisfactory for the process of the present invention. For example, U.S. Pat. No. 5,536,893 to Gudmundsson teaches a method for producing gas hydrates in the form of a fluffy powder by spraying chilled liquid water into a cooled gas. PCT Patent Application WO9827033A1 to Heinemann et al. teaches a method for producing gas hydrates by adiabatically expanding a mixture of water and a cooled compressed gas across a nozzle to a lower pressure. Expansion of the mixture atomizes the water and produces solid gas hydrates. PCT Patent Application WO9919282A1 to Heinemann et al. teaches a method for producing gas hydrates in a fluidized bed reactor by conveying a gas phase upward to fluidize a bed of solid particles, while contacting the gas phase with a downwardly flowing chilled liquid water phase. All of the above-recited methods for producing gas hydrates are relatively inefficient because gas hydrate formation is an exothermic reaction and the evolution of latent heat in the reaction undesirably limits conversion. The above-recited methods require substantial preliminary sub-cooling of the feed streams or large adiabatic pressure drops, both of which substantially increase the cost and complexity of practicing the method.
It is an object of the present invention to provide an effective process for transporting a wet gas, while substantially avoiding pipeline occlusion due to solid gas hydrate accumulation. More particularly, it is an object of the present invention to provide a process for efficiently converting a wet gas to a gas hydrate slurry, wherein the gas hydrate slurry is suitable for transport through a subsea gas transport pipeline. It is another object of the present invention to provide a process for transporting a gas hydrate slurry through a subsea gas transport pipeline, wherein the process is relatively insensitive to the length or depth of the subsea pipeline. It is another object of the present invention to provide a process for converting a wet gas to a gas hydrate slurry by cooling the wet gas with a phase-change heat transfer medium in a heat exchanger, wherein the condenser for the heat transfer medium associated with the heat exchanger is relatively energy efficient. These objects and others are achieved in accordance with the invention described hereafter.
The present invention is a process for transporting a wet gas through a subsea pipeline. The process employs a heat exchanger submersed in a body of water proximal to a subsea well head. The heat exchanger has a first heat exchanger flowpath for a heat transfer medium at an initial lower temperature and a second heat exchanger flowpath for a hydrocarbon stream at an initial higher temperature. The hydrocarbon stream, which is typically a natural gas production stream from an offshore well, includes a gas phase comprising a gaseous hydrocarbon component and an initial liquid phase comprising a liquid aqueous component. The first heat exchanger flowpath has a first inlet and a first outlet and the second heat exchanger flowpath has a second inlet and a second outlet. The first heat exchanger flowpath is separated from the second heat exchanger flowpath by an exchanger heat transfer surface in heat conductive communication with the first and second heat exchanger flowpaths.
A liquid phase portion of the heat transfer medium is conveyed from the first inlet through the first heat exchanger flowpath toward the first outlet. Contact between the exchanger heat transfer surface and the heat transfer medium raises the initial lower temperature of the heat transfer medium to a vaporization temperature and converts the liquid phase portion of the heat transfer medium to a vapor phase portion.
A solid particle medium is entrained within the hydrocarbon stream to form a fluidizable mixture, which is conveyed through the second heat exchanger flowpath. Contact between the exchanger heat transfer surface and the hydrocarbon stream lowers the initial temperature of the hydrocarbon stream to a gas hydrate formation temperature and converts a portion of the hydrocarbon stream to a plurality of solid gas hydrate particles. The solid particle medium minimizes buildup of the solid gas hydrate particles on the exchanger heat transfer surface. The plurality of solid gas hydrate particles and remaining portion of the initial liquid phase in combination form a gas hydrate slurry. The gas hydrate slurry is withdrawn from the second heat exchanger flowpath via the second outlet and fed to a submersed transport pipeline. The transport pipeline extends from the heat exchanger, which is at a lower depth in the body of water, to a desired destination, which is at a higher depth in the body of water. As the gas hydrate slurry is conveyed through the transport pipeline from the lower depth to the higher depth, the temperature of the gas hydrate slurry rises, which inhibits the formation of additional solid gas hydrate particles within the transport pipeline.
A condenser, likewise submersed in the body of water, is provided in association with the heat exchanger. The condenser has a condenser inlet, a condenser outlet and a condenser flowpath angling downward from the inlet to the outlet. The condenser flowpath is enclosed by a condenser heat transfer surface, which is in heat conductive communication with the condenser flowpath. The body of water immersing the condenser heat transfer surface has an ambient temperature substantially less than the vaporization temperature of the heat transfer medium. The vapor phase portion of the heat transfer medium is conveyed from the first outlet of the first heat exchanger flowpath into the condenser inlet and through the condenser flowpath toward the condenser outlet. Contact between the condenser heat transfer surface and the heat transfer medium cools the heat transfer medium to a temperature below the vaporization temperature and reconverts the vapor phase portion to the liquid phase portion. The liquid phase portion is conveyed back from the condenser outlet to the first inlet of the first heat exchanger flowpath.
The invention will be further understood from the accompanying drawings and description.